Process of recovering gold and silver from cyanid solutions by electrolysis.



Patented Feb. 6, 1900.

s. B. cHmsw. PROCESS OF BEGOVEBING GOLD AND SILVER FROM CYANID SOLUTIONS BY ELECTRQLYSIS.

(Application filed Feb. 9, 1898.)

(No Model.)

UNITED STATES PATENT OFFICE.

SAMUEL B. CHRISTY, OF BERKELEY, CALIFORNIA.

PROCESS OF RECOVERING GOLD AND SILVER FROM CYANID SOLUTIONS BY ELECTROLYSIS.

SPECIFICATION forming part of Letters Patent No. 643,096, dated February 6. 1900. Application filed l 'ebrnary 9,1898 Burial 10.369.615. (R0 specimens- To all whom it may concern:

Be it known that I, SAMUEL B. CHRIs'rY, a citizen of the United Statesmesiding at Berkeley', in the'county of Alameda and State of California, have invented certain new and useful Improvements in Electrolytic Recovery of Gold and Silver from Cyanid Ore-Extraction Solutions; and I do hereby declare the following to be a full, clear, and. exact description of the same. v

My invention relates to the metallurgy of the precious metals, and more particularly to the electrolytic precipitation of gold and silver from the large volumes of dilute solutions of alkaline cyanids which result from the extraction of gold and silver ores, tailings, concentrates, and similar products. Such solutions are usually very dilute and offer great resistance to the passage of the electric current. Hence electrodes, large in area and great in number, must be employed to reduce the electric resistance of the solution and to render the operation practicable;

but this creates another difficulty quite as se-' rious-namely, the deposited gold is distributed over an enormous area and in an extrcmely thin film. To make clear the serious nature of this difficulty, I cite the following as an example: 1 suppose-an electrolytic cell or deposition-box containing sixty-one anode and s xty cathode plates, each of ten square feet, or, counting b'oth faces of each cathode, twenty square feet, of effective surface for each cathode, or in all a total of twelve hun-' dred square feet of cathode area for the sixty cathodes. In such a cell an electrodeposition of six hundred ounces gold per month would be regarded as a large output. Now six hundred ounces of gold distributed over an area of" twelve hundred square feet of cathode would make a film only 0. 00034 of an inch or about one three-thousandth of an inch thick. When it is remembered that such a cell often has a cathode area of double what I supposed to deposit less than two hu'ndred'troy ounces per month, it must be evident that the thickness of the gold film will not in'this case excoed one twenty-thousandth of an inch in thickness. It must then be evident how sorious is the nature of the difficulty which remains after the gold is deposited from these dilute oro solutions in a thin film on the oathodes. The precious metal still remains to be recovered from a surface of from twelve hundred to two thousand four hundred square feet of cathode.

In order to make clear the advantages of my invention, it is .only necessary to briefly consider the methods now in use to recover this thin film of goldfrom the cathodes.

Among these the most prominent may be mentioned that of Siemens and Halske,where the gold is deposited on sheets or shavings of lead, which act as cathodes. On these are deposited during the month from six to eight ounces of gold to every one hundred ounces of lead. Hence when the clean-up is made at the end of the month an amount of lead from twelve to sixteen times as great as that of the gold has to be burned or cnpelled off in the cnpellation-furuace. This is an expensive and unsatisfactory method of recovering the gold from the cathodes, thoughit' is at the present time probably the best in use.

In another promment methcd that of Felata'n-Clericithe gold is electrically deposited in amass of quicksilver many times greater in weight than the gold. From the quicksilver the gold can be recovered only after the tedious and unsatisfactory operations of til tering, straining, retorting, and melting.

- In the Keith process, recently introduced,

the gold is precipitated with quicksilver as an amalgam on cathodes of copper or iron. From the enormous area of twelve hundred to two thousand four hundred square feet of cathode the amalgam must be recovered by the tedious and imperfect process of scraping followed by rctortihg and melting.

Still another. method, the Bottel process, recently introduced inSouth Africa, is to precipitate the gold electrolytically on zinc shavings, from which the zinc is subsequently removed by dissolving it in a stronger solution of cyanid of potassium containing gold.

The large number of methods that have been devised for recovering the gold from the cathodes after it has been deposited upon It is the object of my invention to avoid all the difiicultics that come from the handling of added base metalssuch as lead, copper,

qu cksilver, or z nc-and to produce a pure article of gold or silver, or both combined, alloyed only with traces of such metals as may be accidentally contained in the cyanid solution, being simultaneously extracted byit from the ore with the gold and silver. B avoiding an excessive tension at the electrodes of the cell the precipitation of these impurities with the'gold and silver may also to ablar 'eextent be avoided.

Briefl y stated, my process is a process of progressive electro concentration and recov-" cry of the extremely minute traces of gold and silver contained in the dilute cyanid so-.

lutions which result from the extraction of gold and silver ores, concentrates, and tailings.

My process consists, first, in the electrodeposition of gold and silver contained in such dilute solutions upon removable cathodes Tsufiiciently numerous and large in area to secure efiicient deposition; second, in stripping ,and concentrating the thin film of gold and silver thus distributed over a large number and area of cathodes in a smaller volume of cyanid solution'by suitable means. These means for stripping are, in preference, electrblytical by making the gold-laden cathodes in turn anodes in 'thesmaller volume of cyanid solution; but the stripping may also be accomplished by the action of a smaller volume of 'cyanid solution containing suitable oxidizing agents, and, third, in precipitating the gold contained in this smaller volume of enriched cyanid solution electrolytically upon a smaller number of cathodes, preferably upon I one cathode. In general I prefer to perform the second and third operations simultaneonsly; but they may also be separately performed. These general steps, together with all necessary details of apparatus and manipulation, will be more fully understood from the following description and by reference to the accompanyingdrawings, designed to illustrate generally an apparatus suitable t the purpose. r

In the drawings, Figure 1 is'a vertical longitudinal sectional view of what I may term the deposition-box. It is an ordinary electrodeposition-cell in which the gold and silver 'are precipitated from the dilute ore solution and distributed over the enormous area of the cathodes necessary for effective deposition. Fig. 2 is a similar sectional'view at right angles to the planes of the electrodes of what may be termed the linalstripping and concentration box, orsimply the clean-up box for thep irpose of finally concentrating the gold spread out over a large number of cathodes upon a single cathode. Fig. 3 is a similar section of an, intermediate stripping and concentration box.

In Fig. 1,8 represents a water-tiglrt'box provided with grooves on the longitudiual'side's to receive the electrodes A and Q. The anode plates, (marked A,) beginning atthe head of the box, are each slipped into-alternate the anodes are now slipped the cathodes,

(marked 0.) These reach above the surface of the solution and within about an inch ormore of the bottom of the cell, and the cathodes are similarly placed'in electrical connection with the negative pole of thesame shunt wound dynamo or other external source of electric energy. I have not thought it necessary to encumber the drawings with the details of these connections, as they are well understood, deeming it sutiicient to mark the anodes with a plus sign and the cathodes with a minus sign. The ore solution enters at the head of the deposition-box at D and passes down between the first anode and cathode, up between the first cathode and second anode,

and so on alternately up and down in the direction of the arrows till it finally leaves the tail of the box at E. If desired, the circulation of the solution may be equally controlled by having the cathodes reach to the bottom of the box and so as to be immersed below the surface of the solution at the top, while the anodes are lifted above this bottom and above the solution at the top; but in general I prefer the former arrangement. Both electrodes, and in particular thecathodes, are made so that they can bereadily removed one or more at a time from the cell without distu rbing its operation. The anodes and cathodes may be made of any electrocouducting material not too strongly acted on by the solution, such as lead or iron; but usually on account of its cheapness I prefer to make them both of wrought-iron, the anodes about an eighth 'of an inch or less in thickness and the cathodes as thin as they can be made without buckling.

As my claim is for a. process and not for a particular form of apparatus, I do not wish to be understood as confining myself to the particular form of deposition-box here described. Any form of electrolytic cell may be used in which the gold and silver bearing solution is brought into progressive contact with removable cathodes sufiiciently numerous and large in area to secure efficient deposition.

In Fig. 2 is shown a vertical section of-the final stripping and concentration box or the clean-up box, taken at right angles to the plane of the electrodes. F is a water-tight )()X of non-conducting material and containing, preferably, two anodes II 11, en to be connected in parallel with the plus or positive pole of a shunt-wound dynamo or other external source of electric energy, and preferably a single cathode, (marked G,) supposed to be connected with the negative pole of the same source of electric curpposed rent as the anodes H II. Both anodes are made so that they can be removed one at a time from the box F without interfering with the passage of the current. The cathode G is also removable. The anodes H H in the clean-up box F are the gold or silver laden cathodes C C from the depositionbox 13, which are to be stripped of the film which has been deposited upon them. The cathode G in the clean-up box F may be made of any suitable electroconduct-ing material; but in preference I mean extremely thin sheet or series of thin strips of gold or silver of suitable area. This general description will suffice to explain the process carried out in such an apparatus. The solution to be precipitated as it comes from the extraction of an ore is a double cyanid of gold and silver-and an alkali, such as potassium, together with some eyanids of the base metals, also removed by the solution from the ore. The solution should be decidedly alkaline, and if it is not already should be made so by the addition of caustic potash, soda, or lime. Between the anodes and cathodes, each connected in parailel, I now maintain a constant electrical potential, preferably between one and a half and two volts, though-I do not confine myself to these tensions. The proper tension is entirely dependent on the amount of free cyanid present. If the amount of free cyanid is low in proportion to the gold present, lower tensions than one and a half volts will sufiice; but if there is a relatively large amount of free cyanid present in proportion to the gold higher tensions, such as two and a half to three volts, are necessary to get ahigh extraction of the gold; butI prefierenotte use these high tensions unless absolutely neces sary, as they lead to large consumption of. the cyanid used. I prefer to work with the lowest tension possible and to increase tlie precipitation by increasing the cathode area rather than the tension. \Vith a c nstant clectricalpotential at the electrodes and a given strength in free cyanid the rate of precipitation of the gold is fixed by the gold content of the solution, by the number and area of the electrodes, their distance apart, and the rate of flow of the solution through the box. The electrodes should be as numerous and as near together as practicable without causing short-r:ircuit-ing. A distance of an inch or an inch and a half is usually as close as it is safe to place them in practice, owing to danger of buckling; but if vertical strips of wood are fastened to the anodes at intervals of about a foot apart the electrodes may be placed as near together as half'au inch without danger from buckling. When the gold and silver conten of the solution has been sufficiently reduced, it is discharged from the deposition-box l3, and after being suliiciently reinforced with cyanid it is again ready to extract a new portion of ore, and so on indefinitely. The gold may be almost entirely removed from the solution in the deposition box B if sufficient time is given; but the first three-fourths of the gold and silver are much more quickly removed than the last fourth, and as the solution is used over and over again it is usually best to shorten the time of the treatment by precipitating only about three-fourths of the gold contained "in the solution as it enters the box. Now, as

the solution flows through the box B with a constant tension at its electrodes, the electric current at the head of the box will be greater than at its tail, because more gold is contained in the solution as it enters than as it leaves the box, and the cathodes at the head of the box will become more rapidly covered with gold than those at the tail end of the box, where the gold has been more reduced. As the cathodes'at the head of the box become coated with gold I remove them successively in pairs from the head of the deposition-box B and make them anodes in the clean-up box F. Of the cathodes C 0 remaining in the deposition-box B, I prefer to move themstepwise from their places up toward the head of the box, and in the place of those left vacant at the tail end of the box I insert other oath ode-plates that have just been stripped of their gold content as anodes in the clean-up box F, and so on in endless course.

In. order to make more clear the advantages of my method over those now in use and without confining myself to the numbers and dimensions here given merelyinillustration, I will suppose the case cited in the introductionviz., an electrolytic depositioncell with sixty cathode-plates with a total effective cathode area of twelve hundred square feet is producing, say, six, hliindred troy ounces of gold permonth of thirty days. In this ease,if it were required to make a monthly clean-up, I would proceed as follows: Every dayI would take two of theoriginal cathodeplates 0 (J from the head of the depositionbox B, and they would be removed to the clean-up box F. I would then move from the tail of the deposition-box toward its head the remaining cathodes C O, and in place of the spaces left vacant at the foot of this box I would insert two cathodes C O that had just been anodes II II in the box F and had just been stripped of their gold content. In this way a pair of original cathode-plates (J 0 would go everyday to the clean-up boxF. This cell would contain a solutionof an alkaline cyanid. and over again continuously and as itis comparatively small in volume, it may be conveniently made without undue expense slightly stronger in cyanid than the original ore solution; but this extra strength is not usually necessary, and the solution should not he made too strong in cyanid, for then while it strips the gold very rapidly from the anodes it becomes extremely dilficult to redeposit the gold from it upon the cathode. The pair of original cathodes G C from the box B are made to replace the already-stripped anodes H H in the As this solution is used over 1 clean-up box F and are themselves made anodes H Hin the box F. From the sides of the anodes H H turncdtoward the secondary cathode G the gold is rapidly stripped by the electric current, and unless the cyanid solution is made too strong an equal amount is deposited on the secondary cathode G. The current should be so graduated that it is deposited on G in a firm coherent coat. If a very strong current is used, the gold is deposited on the cathode Gas 21. fine brown powder, which may be brushedott at intervals and melted down; but I prefer to deposit it as a firm coherent coat to avoid mechanical loss in handling. As soon as the gold is stripped from one side of the plates H H the other side of each plate is turned toward the cathode G and this side of eachplate is stripped in like manner. After being sufficiently stripped of their gold deposit in the clean up box F the anodes H H are again made cathodes C C in the deposition-box B to receive a new deposit of gold, as before.

While it is desirable to strip the gold from the anodes H H in the clean-up box F as completely as possible at each operation, it is not absolutely necessary to do so, for what is not recovered at one operation will be recovered at the next, as the same plate is alternately cathode O in the box B and anode H in the box F. I

Owing to the fact that the gold dissolved from the anodes II II furnishes as much energy as. is required to deposit the same amount of gold on the cathode G and as on account of the small volume of the solution in the cleanup box it may without undue expense be made slightly stronger in cyanid than that in the deposition-box B, it follows from either or bothof these two causes that the same electromotive force at the dynamo will prod uc a denser current and the plates. H II will he usually stripped clean of their gold in much less than twenty-four hours, so that all the gold deposited in thirty days slow work on the two electrodes II II (called 0 0 when in the box-B) is in one day transferred to the single plateG. By repeating this process daily for thirty days at the end of the month all the gold deposited in thirty days on both sides of thesixty cathodes in the deposition-box B has been transferred in the clean-u p box to th single cathode G. Thus in the case citedinstead of having at'the end of the month six hundred troy ounces of gold distributed over an area of twelve hundred square feet in a fihn one three-thousandth of an inch thick it is concentrated in a single sheet of not more than ten.- sq'u-a'r'e' feet and not less .than one twenty-fifth of an inch in thickness. In such a state it is not neeessaryto melt it into iars. All that is left to be done is to out it into sheets 011asuitable size for transportation to the mint; .IIence without the intervention of-an expensive and complicated auxiliary plant' jentirely foreign in,. na'ture and operation to that used in depositing the gold from the orestripping and concentration box.

solution and by the use of veri simple appliances in harmony with the res of the process I am able to turn out mymonthly product of gold and silver ata nominal extra expense nearly pure and ready for the mint.

In case it has been necessary to use two thousand four hundred square feet' instead of twelve'hundred square feetof cathode-surface in the deposition-box B the advantage of my process over the usual methods is even more apparent. It should be further rcmarked that while after stripping the anodes H II in the clean-up box F it is best to replace the same as cathodes C O at the bottom of the deposition-box B and to move them as they become coated with gold progressively toward the head of the box B until they are finally removed to the clean-up box F this progressive rotation may be omitted and the cathodes O 0 may be removed, stripped, and

returned in any order that may be convenient. In general in spite of the labor involved in the progressive rotation "I prefer it, for when the stripped cathodes C C are placed at the tail of the box they offer no back electromotive force to the current, and the solution, already low in gold at this point, is more completely deposited on the recentlystripped cathodes than itwould be on-cathodesalready partly coated. It is also'evident that I may modifyfthe .relative periods during which any single cathode is being covered, according'to the relative strengths of solutions that are being treated in the depositionbox, without in any way altering the principle and that any person skilled in the electrometallurgic art would be able to make such changes without further instructions. In case, however, of an establishment operating a large number of deposition-boxes then'between these boxes B B and the clean-up box F, I prefer to introduce a third box J, (shown in Fig. 3,) which I term the intermediate The intermediate box thus introduced contains a smaller number of electrodes than the deposition-box B. These anodes K and the secondary cathodes L are all exactly like the cathodes in the deposition-box B and are connected in parallel with an external source of electric energy, as in said box. In using the intermediate stripping and concentration box J the cathodes O G from the deposition-boxes B B are made anodes K in the intermediate box J, and the gold is thus transferred to the cathodes L of this box. The stripped anodes K in the intermediate .box J are then made cathodes O 0 at the tail of the deposition boxes-B B. For example, to work conveniently a plant of, say, five depositiomboxes B B there would he, say, ten anodes K K in the intermediate box J. In this way {all the gold from, say, three hundred cathodes G O in the live deposition-boxes B B could-be concentrated :on, say, eleven cathodes L --L in the intermediate i stripping and; concentration.

box J. Of those eleven cathodes ninewould rco . contain at the end of the month, in the case be intermittently in said intermediate box J receiving their coat of gold. Two others, meanwhile in rotation, would be made amides H H'in the clean-up box F, where their gold content would be transferred to the single sheet or final cathode G, which would thus 'mentioned, of gold or silver, may be made of any other metal, such as iron orlead orcopper. In this case a coat of eleet-rotypers graphite or other similar substance is to be given of potassium.

to the smooth surface of the metah'so that after the gold or silver has been depositedon the same of suificient thickness It may be peeled off in sheets and cut up or melted down, as desired.

Now, although I prercr the electrical strip-" ping and concentrationof the gold and silver in the clean-up box at a single operation, as already described, as the simplest and most efficient method'I do not confine myself to this method, for the gold-laden cathodes O C from the deposition-box ll may be treated one or more at a time in a small box having in it a smaller volume of an alkaline-cyauid solution, together with some suitable oxidizing agent-such as air, peroxids of hy-' drogen, barium, sodium, or some other metal or ferricyanid of potassium, or permanganate mersed in such a solution, so that not only the gold film is immersed, but also the un-,

coated sheet-iron above the gold, the latter will be rapidly dissolved, particularly if the solution is gently agitated, and after stripping a large number of cathodes the enriched solution which finally begins to act slowly is precipitated as before by introducing an anode and a suitable cathode and connecting them with an external source of electrical energy, so that the gold is recovered as before in a single sheet.

I am aware that to deposit gold electrolytically upon cathodes is not ew, nor do I wish to be understood as claiming it to be new to strip films of electroplated metals from electrodes in a conducting solution by electrical means, such being employed by electro- If the cathodes O C are implaters torecover the gold from imperfectlyvplated articles. Iam also aware that British Patent No. 16,737, granted Pfleger September 6, 1895, suggests that by placing the gold plated electrodes in a ten-per-cent. cyanidof-potassium solution in contact with a plate of carbon the gold will dissolve and a concentrated solution will result, fromwhich the gold may afterward be extracted. 1 am also redeposit this gold electrolytically upon suitable cathodes, this practice being long known to electroplaters,as shown by Wahl, p. 260,

aware that it is not new to strip this gold films from imperfectly-plated articles and to paragraph 2, where a ten-per-cent. solution of cyanid is used. None of these separate operations do I claim to be new and expressly disclaim same. However, so far as I am aware such steps have never before been combined and coordinated in a single progressive process in such manner that they constitute a common function, whereby the extremely minute traces of gold and silver- (often less than one two-hundreths of one per cent.) contained in the extremely dilute cyanid solutions which result from the treat-.

ment of ores, concentrates, and tailings have been concentrated by electrolytical methods 7 alone in a single sheet of pure metal, thus securing a concentration of more than two hundred thousand to onewithout resorting to any auxiliary'method or operation.

What I claim, and desire to secure by Let ters Patent, is-

The process'of progressive electroconcentration and recovery of gold and silver contained in the large volumes of dilute cyanid solutions containing free alkali resulting from the extraction of gold and silver ores, tailings and concentrates-which consists-first,

in depositing the gold and silver electrolytically from said solution upon removable cathodes sufficiently numerous and large in area to secure efijcient deposition, and second, in making said removable original cathodes'suecessively anodes in a smaller vgolume of cyanid solution and transferring and depositing electrolytically the thin film of gold and silver, already distributed over a large number of said original cathodes, upon a smaller number of secondary cathodes, also contained in said smaller volume oteyanid solution.

"In witness whereof 1 have hereunto set my hand.

SAMUEL B. CHRISTY. Witnesses:

D. B. RICHARDS,

WALTER F. VANE. 

